1. Field of the Invention
The invention generally relates to a method for blowmolding a container, and more particularly to a method for blowmolding a container with a base having sufficient rigidity to withstand forces experienced during inversion of a base wall.
2. Related Art
One method of manufacturing containers is through a process known as stretch blowmolding. In this process, a preformed parison, or preform, is prepared from a thermoplastic material, typically by an injection molding process. The preform typically includes a threaded end, which becomes the threads of the container. During stretch blowmolding, the preform is positioned between two open blow mold halves. The blow mold halves close about the preform and cooperate to provide a cavity into which the preform is blown to form the container. Once the mold is closed, a gas is forced into the perform causing it to stretch and to take the shape of the mold as the plastic contacts the mold. After molding, the mold halves open to release the blowmolded container.
One problem with stretch blowmolding is that stretching of the plastic material may affect the performance of the container at certain areas. While the stretching of the plastic material may not cause problems for most sections of the container, it particularly affects the ability of the plastic material to form around a deep protrusion in the mold. In some applications of container manufacturing, a deep protrusion may be required at a particular section of a container, most often at a base of the container. As the plastic contacts the deep protrusion of the mold, the plastic must stretch and flow around the protrusion into a recess. However, the plastic material is less able to flow and stretch around the protrusion because of the contact friction with the mold surface. Insufficient material distribution at a region, such as at the base, may affect the ability of the region to maintain its shape around the protrusion during hot filling, the strength of the region, or the ability of the container to stand on a flat surface.
A lack of definition in the base caused by the inability of the plastic to properly form at a deep protrusion is a particular problem when one or more invertable walls are included in the base. If the invertable walls or the surrounding areas of the base are not sufficiently rigid, inversion of the walls may deform the base, which may cause problems in the ability of the container to stably stand on a flat surface and may affect the appearance of the container.
What is needed is an improved method of forming a container base that overcomes the shortcomings of conventional solutions.